A. Film Treatment and Metal Forming Process
Flexible metal-film laminate materials have become the focus of great interest in recent years. A variety of techniques including film pre-treatment processes and metal layer forming processes have been studied to improve the properties of laminates. Such processes include film etching, organic coatings, the sputtering of metal, the adhesive lamination of pre-formed metal foil to film, electroless plating, electroplating, and others. Such metal-film laminate products each have different characteristics including metal thickness, metal tensile strength, laminate flexibility, delamination tendency, etc.
One useful metal forming method involves producing a "tie coat" of chromium typically on a polyimide surface. Such tie coats are equal to or greater than 50 nanometers, greater than 100 atomic layers, of chromium atoms, in thickness and aid in bonding a electroplated copper layer to polyimide. We are aware that a successful film laminate comprises a film having a greater than 50 nanometer thickness (more than 100 atom monolayers) chromium tie coat and a plated copper layer having a thickness of greater than 0.1 .mu.m. Such laminates are inconvenient to process into wiring boards because they generally require two etch steps, one basic etch for copper and a second acid etch for chromium. Polyimide laminates with a "tie coat" have acceptable peel strength but can be expensive to produce and use in applications requiring low cost materials.
Kennedy, U.S. Pat. No. 3,700,538 discloses an adhesive used to bond copper foil to resin impregnated fiber glass cloth using a polyimide resin adhesive. The use of an adhesion promoter to bond metal to an insulating base material is known. For example, Soukup et al., U.S. Pat. No. 3,477,900 and Goepfert et al., U.S. Pat. No. 3,149,021 discloses that when the insulating base material comprises methylmethacrylate resin, an unsaturated polyester may be added to the resin as an adhesion promoter to bond a copper foil. However, these patents disclose that an increase in the proportion of polyester is generally accompanied by a decrease in adhesion of the copper foil to the resinous base. Barrell et al., U.S. Pat. No. 4,420,509 and Cordts et al., U.S. Pat. No. 4,093,768 disclose procedures for preparing polyester resin copper clad laminates. These processes require several steps or expensive continuously operating equipment.
Van Essen, U.S. Pat. No. 4,081,578; Shanoski et al., U.S. Pat. No. 4,189,517 and Cobbledick et at., U.S. Pat. No. 4,414,173 are directed to in-mold coating processes which are substantially different from the present process in that a preform substrate is either made or placed in a mold and cured. The mold is opened and a small amount of resin is placed on the molded substrate sufficient to form a coating up to about 20 mils. in thickness. The mold is then closed over the polymerizing resin to apply pressure
Japanese Patent No. 57-83427 discloses a process where an insulation material is mounted on an inner surface of an injection mold and a metal foil is overlaid on the insulated surface and fixed. A thermoplastic resin is melt-injected into the mold to provide a resin product laminated firmly with the metal foil.
Bristowe et al, U.S. Pat. No. 4,916,016 also teaches injection molded metal-thermoset laminates.
Kawakami et al, U.S. Pat. No. 4,913,938 teaches coating a resin substrate with a copper solution and heating in a non-oxidizing atmosphere to increase copper laminate adhesion.
Pinch et al, U.S. Pat. No. 4,772,488 teaches the use of a carbon dioxide plasma to treat and clean dielectric layers.
Haque et al, U.S. Pat. No. 4,524,089 uses a three step plasma treatment of copper foils.
Shanefield et al, U.S. Pat. Nos. 4,444,848 and 4,582,564 teach a sputter etching of a rubber modified epoxy surface or coating.
Holmes et al, U.S. Pat. No. 4,153,518 teaches treating a refractory metal oxide layer to improve adhesion of oxide forming metals.
Toth et al, U.S. Pat. No. 4,568,413 teaches forming a releasable metallic layer on a polymeric carrier, adhering the releasable metal onto a substrate and peeling the carrier.
Sato, U.S. Pat. No. 4,193,849 teaches conventional pre-treatments of plastic prior to electro-chemical deposition of metal surfaces.
Ho et al, U.S. Pat. No. 4,720,401 teaches heating a film substrate to a temperature between 0.6 and 0.8 of the curing temperature (T.sub.C) of the substrate material, commonly an elevated temperature exceeding 200.degree. C. (often 240.degree.-280.degree. C.) and evaporating or sputtering metal ions such that a metal ion can interact with the heated substrate layer and penetrate into the interior of the heated substrate. The processes in Ho et al are done in an inert atmosphere and produce no metal oxide.
Fronz et al, Plasma Pretreatment of Polyimide Films, a paper presented at the Apr. 24-28, 1989 meeting of the Soc. of Vacuum Coaters, teach many of the drawbacks of copper-polyimide laminates. Fronz et al teaches that surface cleaning of the polyimide film will increase peel strength. Fronz et al does not discuss the importance of metal-oxide adhesion structures nor uses metallic methods in the film treatment.
One pre-treatment technique used with films called "corona discharge" has been found to aid in surface cleaning but not helpful in introducing binding structures to promote the peel strength of laminates. Corona discharge uses ceramic elements or other types of non metallic electrode and an ambient atmosphere temperature/pressure discharge to generate UV radiation and ozone (O.sub.3). This treatment apparently produces no new metal/film metal-oxide/film structure on the surface of the film and does not appear to promote film-laminate bonding.
While film treatment steps and metal forming processes are known, a fully satisfactory laminate has not been prepared. In particular, no fully satisfactory polyimide laminate is known for use in printed circuit board manufacture.
B. Delamination Tendency of Laminates
In general, metal-film laminates, which can be formed by forming (plating) metal onto the film or onto treated film, have a tendency to delaminate during and after the formation of a plated metal layer having a thickness of about 0.1 to 35 .mu.m. The peel strength of many such laminates currently in use is generally felt to be insufficient for many end uses because any delamination can cause the failure of the laminate to operate in its intended use. However, even the peel strength currently achievable in many films can be still further decreased by exposure of the film to processing chemicals (etchants, cleaners, coatings, etc.) and environmental stress (such as humidity in the case of polyimide) and can be reduced to much less than 3 pounds per inch and in certain instance, can be much less than 1 pound per inch. Delamination of the metal layer can result in the failure of the material to be reflective, insulating, an adequate packaging material or to function in a useful circuit assembled on a printed wiring board made from the laminate.
A variety of other influences can promote the delamination of metal poorly bonded to film substrate. First, the strength of the laminate bond is an important characteristic. Higher strength bonds reduce delamination tendency. Further, the mechanical stresses (soldering, film flex during processing, etc.) involved in first forming the metal on the flexible film and in subsequent processing steps can cause the film to distort or flex and can cause the poorly bonded metal to leave the film.
Additionally, a number of polymer surfaces are known to be less likely to maintain an integral laminate structure. Fluorocarbon resins, polyethylene, polypropylene, and polyvinylidiene chloride or polyvinylidiene-fluoride films tend to be difficult surfaces for metal bonding.
Flexible printed circuit boards are currently one preferred circuit manufacturing format used in a variety of electronic devices. These boards are fabricated from flexible plastic substrates having a thin copper metal laminate layer and can have conductive metal on one or both surfaces with through-hole interconnections. During circuit fabrication, copper is selectively removed by chemical etching or is pattern plated to leave a pattern of the desired interconnecting circuitry between various components in an electronic circuit. With improvements in etching technology, intercircuit line spacings approaching two-thousandths of an inch can be achieved. Narrow line spacing is one of the current technical innovations that permit continued miniaturization of complex circuitry. However, a narrow line width can promote delamination.
As a result of the problems in laminate preparations and the rigors of the laminate use, an increase in the bond strength of the metal layer to the film polymer substrate is a highly desirable end in the production of inexpensive delamination resistent metal-film laminates. Further, production of low cost polyester laminate and low cost polyimide laminate, free of a "tie coat", has not been achieved despite a long felt need.
Accordingly, a substantial need exists for delamination resistant metal-film laminates and for processes for the preparation of such laminate materials from the film materials including polyester and polyimide. A further need exists to form delamination resistant metal-film laminates on hard to bond films. The preferred laminates are substantially resistant to delamination caused by either chemical treatments or mechanical stresses.